seaweedfs/weed/s3api/auth_signature_v4.go

/*
 * The following code tries to reverse engineer the Amazon S3 APIs,
 * and is mostly copied from minio implementation.
 */

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
// implied. See the License for the specific language governing
// permissions and limitations under the License.

package s3api

import (
	"bytes"
	"context"
	"crypto/hmac"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/hex"
	"io"
	"net"
	"net/http"
	"net/url"
	"regexp"
	"slices"
	"sort"
	"strconv"
	"strings"
	"time"
	"unicode/utf8"

	"github.com/seaweedfs/seaweedfs/weed/glog"
	weed_iam "github.com/seaweedfs/seaweedfs/weed/iam"
	"github.com/seaweedfs/seaweedfs/weed/iam/sts"

	"github.com/seaweedfs/seaweedfs/weed/s3api/s3_constants"
	"github.com/seaweedfs/seaweedfs/weed/s3api/s3err"
)

func (iam *IdentityAccessManagement) reqSignatureV4Verify(r *http.Request) (*Identity, s3err.ErrorCode) {
	switch {
	case isRequestSignatureV4(r):
		identity, _, errCode := iam.doesSignatureMatch(r)
		return identity, errCode
	case isRequestPresignedSignatureV4(r):
		identity, _, errCode := iam.doesPresignedSignatureMatch(r)
		return identity, errCode
	}
	return nil, s3err.ErrAccessDenied
}

// Constants specific to this file
const (
	emptySHA256                   = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
	streamingContentSHA256        = "STREAMING-AWS4-HMAC-SHA256-PAYLOAD"
	streamingContentSHA256Trailer = "STREAMING-AWS4-HMAC-SHA256-PAYLOAD-TRAILER"
	streamingUnsignedPayload      = "STREAMING-UNSIGNED-PAYLOAD-TRAILER"
	unsignedPayload               = "UNSIGNED-PAYLOAD"
	// Limit for IAM/STS request body size to prevent DoS attacks
	iamRequestBodyLimit = 10 * (1 << 20) // 10 MiB
)

// streamHashRequestBody computes SHA256 hash incrementally while preserving the body.
func streamHashRequestBody(r *http.Request, sizeLimit int64) (string, error) {
	if r.Body == nil {
		return emptySHA256, nil
	}

	limitedReader := io.LimitReader(r.Body, sizeLimit)
	hasher := sha256.New()
	var bodyBuffer bytes.Buffer

	// Use io.Copy with an io.MultiWriter to hash and buffer the body simultaneously.
	if _, err := io.Copy(io.MultiWriter(hasher, &bodyBuffer), limitedReader); err != nil {
		return "", err
	}

	r.Body = io.NopCloser(bytes.NewReader(bodyBuffer.Bytes()))

	if bodyBuffer.Len() == 0 {
		return emptySHA256, nil
	}

	return hex.EncodeToString(hasher.Sum(nil)), nil
}

// getContentSha256Cksum retrieves the "x-amz-content-sha256" header value.
func getContentSha256Cksum(r *http.Request) string {
	// If the client sends a SHA256 checksum of the object in this header, use it.
	if v := r.Header.Get("X-Amz-Content-Sha256"); v != "" {
		return v
	}

	// For a presigned request we look at the query param for sha256.
	if isRequestPresignedSignatureV4(r) {
		// X-Amz-Content-Sha256 header value is optional for presigned requests.
		return unsignedPayload
	}

	// X-Amz-Content-Sha256 header value is required for all non-presigned requests.
	return emptySHA256
}

// normalizePayloadHash converts base64-encoded payload hash to hex format.
// AWS SigV4 canonical requests always use hex-encoded SHA256.
func normalizePayloadHash(payloadHashValue string) string {
	// Special values and hex-encoded hashes don't need conversion
	if payloadHashValue == emptySHA256 || payloadHashValue == unsignedPayload ||
		payloadHashValue == streamingContentSHA256 || payloadHashValue == streamingContentSHA256Trailer ||
		payloadHashValue == streamingUnsignedPayload || len(payloadHashValue) == 64 {
		return payloadHashValue
	}

	// Try to decode as base64 and convert to hex
	if decodedBytes, err := base64.StdEncoding.DecodeString(payloadHashValue); err == nil && len(decodedBytes) == 32 {
		return hex.EncodeToString(decodedBytes)
	}

	return payloadHashValue
}

// requestSigningService extracts the service name from the SigV4 credential
// scope (…/region/service/aws4_request) so a failed request can be answered in
// that service's error envelope.
func requestSigningService(r *http.Request) string {
	credential := r.URL.Query().Get("X-Amz-Credential")
	if credential == "" {
		v4Auth := strings.Replace(r.Header.Get("Authorization"), " ", "", -1)
		v4Auth = strings.TrimPrefix(v4Auth, signV4Algorithm)
		for _, field := range strings.Split(v4Auth, ",") {
			if cred, ok := strings.CutPrefix(field, "Credential="); ok {
				credential = cred
				break
			}
		}
	}
	if parts := strings.Split(credential, "/"); len(parts) == 5 {
		return parts[3]
	}
	return ""
}

// signValues data type represents structured form of AWS Signature V4 header.
type signValues struct {
	Credential    credentialHeader
	SignedHeaders []string
	Signature     string
}

// parseSignV4 parses the authorization header for signature v4.
func parseSignV4(v4Auth string) (sv signValues, aec s3err.ErrorCode) {
	// Replace all spaced strings, some clients can send spaced
	// parameters and some won't. So we pro-actively remove any spaces
	// to make parsing easier.
	v4Auth = strings.Replace(v4Auth, " ", "", -1)
	if v4Auth == "" {
		return sv, s3err.ErrAuthHeaderEmpty
	}

	// Verify if the header algorithm is supported or not.
	if !strings.HasPrefix(v4Auth, signV4Algorithm) {
		return sv, s3err.ErrSignatureVersionNotSupported
	}

	// Strip off the Algorithm prefix.
	v4Auth = strings.TrimPrefix(v4Auth, signV4Algorithm)
	authFields := strings.Split(strings.TrimSpace(v4Auth), ",")
	malformed := len(authFields) != 3
	var hasCredential, hasSignature bool
	for _, field := range authFields {
		field = strings.TrimSpace(field)
		malformed = malformed || field == ""
		hasCredential = hasCredential || strings.HasPrefix(field, "Credential=")
		hasSignature = hasSignature || strings.HasPrefix(field, "Signature=")
	}
	if malformed {
		// AWS distinguishes which required field is absent.
		if !hasCredential {
			return sv, s3err.ErrInvalidArgument
		}
		return sv, s3err.ErrAuthorizationHeaderMalformed
	}

	// Initialize signature version '4' structured header.
	signV4Values := signValues{}

	var err s3err.ErrorCode
	// Save credential values.
	signV4Values.Credential, err = parseCredentialHeader(authFields[0])
	if err != s3err.ErrNone {
		return sv, err
	}

	// Save signed headers.
	signV4Values.SignedHeaders, err = parseSignedHeader(authFields[1])
	if err != s3err.ErrNone {
		return sv, err
	}

	// Save signature.
	signV4Values.Signature, err = parseSignature(authFields[2])
	if err != s3err.ErrNone {
		return sv, err
	}

	// Return the structure here.
	return signV4Values, s3err.ErrNone
}

// buildPathWithForwardedPrefix combines forwarded prefix with URL path while preserving S3 key semantics.
// This function avoids path.Clean which would collapse "//" and dot segments, breaking S3 signatures.
// It only normalizes the join boundary to avoid double slashes between prefix and path.
func buildPathWithForwardedPrefix(forwardedPrefix, urlPath string) string {
	if forwardedPrefix == "" {
		return urlPath
	}
	// Ensure single leading slash on prefix
	if !strings.HasPrefix(forwardedPrefix, "/") {
		forwardedPrefix = "/" + forwardedPrefix
	}
	// Join without collapsing interior segments; only fix a double slash at the boundary
	var joined string
	if strings.HasSuffix(forwardedPrefix, "/") && strings.HasPrefix(urlPath, "/") {
		joined = forwardedPrefix + urlPath[1:]
	} else if urlPath == "" {
		joined = forwardedPrefix
	} else if !strings.HasSuffix(forwardedPrefix, "/") && !strings.HasPrefix(urlPath, "/") {
		joined = forwardedPrefix + "/" + urlPath
	} else {
		joined = forwardedPrefix + urlPath
	}
	// Trailing slash semantics inherited from urlPath (already present if needed)
	return joined
}

// v4AuthInfo holds the parsed authentication data from a request,
// whether it's from the Authorization header or presigned URL query parameters.
type v4AuthInfo struct {
	Signature     string
	AccessKey     string
	SignedHeaders []string
	Date          time.Time
	Region        string
	Service       string
	Scope         string
	HashedPayload string
	IsPresigned   bool
}

// verifyV4Signature is the single entry point for verifying any AWS Signature V4 request.
// It handles standard requests, presigned URLs, and the seed signature for streaming uploads.
func (iam *IdentityAccessManagement) verifyV4Signature(r *http.Request, shouldCheckPermissions bool) (identity *Identity, credential *Credential, calculatedSignature string, authInfo *v4AuthInfo, errCode s3err.ErrorCode) {
	// 1. Extract authentication information from header or query parameters
	authInfo, errCode = extractV4AuthInfo(r)
	if errCode != s3err.ErrNone {
		return nil, nil, "", nil, errCode
	}

	var cred *Credential

	// 2. Resolve the credential. A configured access key wins over a session
	// token: credential vendors like Unity Catalog emit a token with static
	// credentials too, and it adds nothing the signature does not prove.
	// STS-issued access keys are never registered in the static map, so they
	// still land on session-token validation.
	sessionToken := extractSessionToken(r)
	var found bool
	identity, cred, found = iam.lookupByAccessKey(authInfo.AccessKey)
	switch {
	case found:
		// Check service account expiration
		if cred.isCredentialExpired() {
			glog.V(2).Infof("Service account credential %s has expired (expiration: %d, now: %d)",
				authInfo.AccessKey, cred.Expiration, time.Now().Unix())
			return nil, nil, "", nil, s3err.ErrAccessDenied
		}
		if sessionToken != "" {
			*r = *r.WithContext(s3_constants.IgnoreSessionTokenInContext(r.Context()))
		}
	case sessionToken != "":
		// Validate STS session token
		identity, cred, errCode = iam.validateSTSSessionToken(r, sessionToken, authInfo.AccessKey)
		if errCode != s3err.ErrNone {
			return nil, nil, "", nil, errCode
		}
	default:
		// Log detailed error information for InvalidAccessKeyId (avoid slice allocation for performance)
		iam.m.RLock()
		keyCount := len(iam.accessKeyIdent)
		iam.m.RUnlock()

		glog.Warningf("InvalidAccessKeyId: attempted key '%s' not found. Available keys: %d, Auth enabled: %v",
			authInfo.AccessKey, keyCount, iam.isAuthEnabled)
		return nil, nil, "", nil, s3err.ErrInvalidAccessKeyID
	}

	// 3. Perform permission check
	if shouldCheckPermissions {
		bucket, object := s3_constants.GetBucketAndObject(r)
		action := s3_constants.ACTION_READ
		if r.Method != http.MethodGet && r.Method != http.MethodHead {
			action = s3_constants.ACTION_WRITE
		}

		// Use centralized permission check
		if errCode = iam.VerifyActionPermission(r, identity, Action(action), bucket, object); errCode != s3err.ErrNone {
			return nil, nil, "", nil, errCode
		}
	}

	// 4. Handle presigned request expiration
	if authInfo.IsPresigned {
		if errCode = checkPresignedRequestExpiry(r, authInfo.Date); errCode != s3err.ErrNone {
			return nil, nil, "", nil, errCode
		}
	}

	// 5. Extract headers that were part of the signature
	extractedSignedHeaders, errCode := extractSignedHeaders(authInfo.SignedHeaders, r, iam.externalHost)
	if errCode != s3err.ErrNone {
		return nil, nil, "", nil, errCode
	}

	// 5a. Reject requests that carry x-amz-* headers outside of SignedHeaders.
	// Without this check a presigned-URL holder can inject unsigned headers
	// (x-amz-tagging, x-amz-acl, x-amz-meta-*, x-amz-storage-class, x-amz-object-lock-*,
	// x-amz-server-side-encryption*, etc.) that downstream handlers still persist.
	if errCode = verifySignedHeadersCoverage(r, authInfo.SignedHeaders, authInfo.IsPresigned); errCode != s3err.ErrNone {
		return nil, nil, "", nil, errCode
	}

	// 6. Get the query string for the canonical request
	queryStr := getCanonicalQueryString(r, authInfo.IsPresigned)

	// 7. Define a closure for the core verification logic to avoid repetition
	verify := func(urlPath string) (string, s3err.ErrorCode) {
		return calculateAndVerifySignature(
			cred.SecretKey,
			r.Method,
			urlPath,
			queryStr,
			extractedSignedHeaders,
			authInfo,
		)
	}

	// 8. Verify the signature for each plausible host value: when X-Forwarded-Host carries
	// no port, the client may have signed the Host header the proxy kept or the forwarded
	// host and port, and the headers alone cannot tell which.
	// A router that rewrote the request path (e.g. the Iceberg catalog's
	// doubled-/v1 collapse) records the client-sent path in the context;
	// the signature covers that original path.
	pathForSignature := s3_constants.GetOriginalRequestPathFromContext(r)
	if pathForSignature == "" {
		pathForSignature = r.URL.EscapedPath()
		if pathForSignature == "" {
			pathForSignature = r.URL.Path
		}
	}
	// The signature is checked against prefix+path but the request is routed
	// and authorized on path alone, so only a trusted proxy may set the prefix.
	iam.m.RLock()
	trustedPeer := iam.trustedProxies.IsTrustedPeer(r)
	iam.m.RUnlock()
	forwardedPrefix := ""
	if trustedPeer {
		forwardedPrefix = r.Header.Get("X-Forwarded-Prefix")
	}
	var matchedHost string
	for i, hostCandidate := range extractHostHeaderCandidates(r, iam.externalHost) {
		if i > 0 && !replaceSignedHostHeader(extractedSignedHeaders, hostCandidate) {
			break
		}

		// 9. Verify with the X-Forwarded-Prefix path first
		if forwardedPrefix != "" {
			cleanedPath := buildPathWithForwardedPrefix(forwardedPrefix, pathForSignature)
			calculatedSignature, errCode = verify(cleanedPath)
			if errCode == s3err.ErrNone {
				matchedHost = hostCandidate
				break
			}
		}

		// 10. Verify with the original path
		calculatedSignature, errCode = verify(pathForSignature)
		if errCode == s3err.ErrNone {
			matchedHost = hostCandidate
			break
		}

		// 11. Retry with decoded path if signature used raw path encoding
		if decodedPath, decodeErr := url.PathUnescape(pathForSignature); decodeErr == nil && decodedPath != pathForSignature {
			calculatedSignature, errCode = verify(decodedPath)
			if errCode == s3err.ErrNone {
				matchedHost = hostCandidate
				break
			}
		}
	}

	if matchedHost != "" {
		// The key of the credential that verified: the audit log and the
		// catalog's vended sessions name it (never a key re-read from the request).
		// Recorded before the retarget check so that refusal names it too.
		s3_constants.RecordAccessKeyId(r.Context(), cred.AccessKey)
		if signedBucket, ok := bucketFromVirtualHost(matchedHost, iam.domain); ok {
			if routedBucket, _ := s3_constants.GetBucketAndObject(r); routedBucket != "" && routedBucket != signedBucket {
				glog.V(2).Infof("reject %s %s: signed host %q implies bucket %q but routed to %q",
					r.Method, r.URL.Path, matchedHost, signedBucket, routedBucket)
				return nil, nil, "", nil, s3err.ErrAccessDenied
			}
		}
		return identity, cred, calculatedSignature, authInfo, s3err.ErrNone
	}

	return nil, nil, "", nil, errCode
}

func replaceSignedHostHeader(headers http.Header, host string) bool {
	replaced := false
	for name := range headers {
		if strings.EqualFold(name, "host") {
			headers[name] = []string{host}
			replaced = true
		}
	}
	return replaced
}

// validateSTSSessionToken validates an STS session token and extracts temporary credentials
func (iam *IdentityAccessManagement) validateSTSSessionToken(r *http.Request, sessionToken string, accessKey string) (*Identity, *Credential, s3err.ErrorCode) {
	// Check if IAM integration is available
	if iam.iamIntegration == nil {
		glog.V(2).Infof("IAM integration not available, cannot validate session token")
		return nil, nil, s3err.ErrInvalidAccessKeyID
	}

	// Validate the session token with the STS service
	ctx := r.Context()
	sessionInfo, err := iam.iamIntegration.ValidateSessionToken(ctx, sessionToken)
	if err != nil {
		glog.V(2).Infof("Failed to validate STS session token: %v", err)
		return nil, nil, s3err.ErrInvalidAccessKeyID
	}

	// Check if sessionInfo is nil
	if sessionInfo == nil {
		glog.Warningf("STS service returned nil session info for token validation")
		return nil, nil, s3err.ErrInvalidAccessKeyID
	}

	// Check if Credentials are nil
	if sessionInfo.Credentials == nil {
		glog.Warningf("STS service returned nil credentials in session info")
		return nil, nil, s3err.ErrInvalidAccessKeyID
	}

	// Validate that credentials have the required access key
	if sessionInfo.Credentials.AccessKeyId == "" {
		glog.Warningf("STS service returned empty AccessKeyId in credentials")
		return nil, nil, s3err.ErrInvalidAccessKeyID
	}

	// Verify that the access key in the request matches the one in the session token
	if sessionInfo.Credentials.AccessKeyId != accessKey {
		// Mask access keys to avoid exposing credentials in logs
		truncateKey := func(k string) string {
			const mask = "***"
			if len(k) > 4 {
				return k[:4] + mask
			}
			return mask
		}
		glog.V(2).Infof("Access key mismatch: request has %s, session token has %s",
			truncateKey(accessKey), truncateKey(sessionInfo.Credentials.AccessKeyId))
		return nil, nil, s3err.ErrInvalidAccessKeyID
	}

	// Check if the session has expired
	if sessionInfo.ExpiresAt.IsZero() {
		glog.Warningf("STS service returned zero/empty expiration time")
		return nil, nil, s3err.ErrInvalidAccessKeyID
	}

	if time.Now().After(sessionInfo.ExpiresAt) {
		glog.V(2).Infof("STS session has expired at %v", sessionInfo.ExpiresAt)
		return nil, nil, s3err.ErrExpiredToken
	}

	// Validate required credential fields
	if sessionInfo.Credentials.SecretAccessKey == "" {
		glog.Warningf("STS service returned empty SecretAccessKey in credentials")
		return nil, nil, s3err.ErrInvalidAccessKeyID
	}

	// Validate principal information
	if sessionInfo.AssumedRoleUser == "" || sessionInfo.Principal == "" {
		glog.Warningf("STS service returned empty AssumedRoleUser or Principal (user=%q, principal=%q)",
			sessionInfo.AssumedRoleUser, sessionInfo.Principal)
		return nil, nil, s3err.ErrInvalidAccessKeyID
	}

	// Create a credential from the session info
	cred := &Credential{
		AccessKey:  sessionInfo.Credentials.AccessKeyId,
		SecretKey:  sessionInfo.Credentials.SecretAccessKey,
		Status:     weed_iam.AccessKeyStatusActive,
		Expiration: sessionInfo.ExpiresAt.Unix(),
	}

	// Create claims map from request context
	// The request context contains user information from the original OIDC token
	// that was used in AssumeRoleWithWebIdentity (e.g., preferred_username, email, etc.)
	claims := make(map[string]interface{}, len(sessionInfo.RequestContext))
	for k, v := range sessionInfo.RequestContext {
		claims[k] = v
	}

	// Key the principal on the OIDC subject so a session resolves to the same
	// identity on the SigV4 and JWT paths (see s3_iam_middleware.go); fall back to
	// the assumed-role user when absent. Distinct per principal, not shared admin.
	principal := sessionInfo.Subject
	if principal == "" {
		principal = sessionInfo.AssumedRoleUser
	}
	identity := &Identity{
		Name:         principal,
		Account:      &Account{Id: principal, DisplayName: sessionInfo.SessionName, EmailAddress: principal + "@seaweedfs.local"},
		Credentials:  []*Credential{cred},
		PrincipalArn: sessionInfo.Principal,
		PolicyNames:  sessionInfo.Policies, // Populate PolicyNames for IAM authorization
		Claims:       claims,               // Populate Claims for policy variable substitution
	}
	// ParentUser is set only for OIDC-federated sessions (see
	// AssumeRoleWithWebIdentity), so it gates the audit identity claim: without
	// it the request context's sub is the opaque session subject injected by
	// ValidateJWTWithClaims, not the OIDC subject, and must not be surfaced as
	// an authoritative identity.
	if sessionInfo.ParentUser != "" {
		identity.IdentityClaim = sts.ResolveIdentityClaim(sessionInfo.RequestContext)
	}

	glog.V(2).Infof("Successfully validated STS session token for principal: %s, assumed role user: %s",
		sessionInfo.Principal, sessionInfo.AssumedRoleUser)
	return identity, cred, s3err.ErrNone
}

// calculateAndVerifySignature contains the core logic for creating the canonical request,
// string-to-sign, and comparing the final signature.
func calculateAndVerifySignature(secretKey, method, urlPath, queryStr string, extractedSignedHeaders http.Header, authInfo *v4AuthInfo) (string, s3err.ErrorCode) {
	canonicalRequest := getCanonicalRequest(extractedSignedHeaders, authInfo.HashedPayload, queryStr, urlPath, method)
	stringToSign := getStringToSign(canonicalRequest, authInfo.Date, authInfo.Scope)
	signingKey := getSigningKey(secretKey, authInfo.Date.Format(yyyymmdd), authInfo.Region, authInfo.Service)
	newSignature := getSignature(signingKey, stringToSign)

	if !compareSignatureV4(newSignature, authInfo.Signature) {
		glog.V(4).Infof("Signature mismatch. Details:\n- CanonicalRequest: %q\n- StringToSign: %q\n- Calculated: %s, Provided: %s",
			canonicalRequest, stringToSign, newSignature, authInfo.Signature)
		return "", s3err.ErrSignatureDoesNotMatch
	}

	return newSignature, s3err.ErrNone
}

func extractV4AuthInfo(r *http.Request) (*v4AuthInfo, s3err.ErrorCode) {
	if isRequestPresignedSignatureV4(r) {
		return extractV4AuthInfoFromQuery(r)
	}
	return extractV4AuthInfoFromHeader(r)
}

func extractV4AuthInfoFromHeader(r *http.Request) (*v4AuthInfo, s3err.ErrorCode) {
	authHeader := r.Header.Get("Authorization")
	signV4Values, errCode := parseSignV4(authHeader)
	if errCode != s3err.ErrNone {
		return nil, errCode
	}

	var t time.Time
	if xamz := r.Header.Get("x-amz-date"); xamz != "" {
		parsed, err := time.Parse(iso8601Format, xamz)
		if err != nil {
			return nil, s3err.ErrMalformedDate
		}
		t = parsed
	} else {
		ds := r.Header.Get("Date")
		if ds == "" {
			return nil, s3err.ErrMissingDateHeader
		}
		parsed, err := http.ParseTime(ds)
		if err != nil {
			return nil, s3err.ErrMalformedDate
		}
		t = parsed.UTC()
	}

	// Validate clock skew: requests cannot be older than 15 minutes from server time to prevent replay attacks
	const maxSkew = 15 * time.Minute
	now := time.Now().UTC()
	if now.Sub(t) > maxSkew || t.Sub(now) > maxSkew {
		return nil, s3err.ErrRequestTimeTooSkewed
	}

	hashedPayload := getContentSha256Cksum(r)
	if signV4Values.Credential.scope.service != "s3" && hashedPayload == emptySHA256 && r.Body != nil {
		var hashErr error
		hashedPayload, hashErr = streamHashRequestBody(r, iamRequestBodyLimit)
		if hashErr != nil {
			return nil, s3err.ErrInternalError
		}
	}

	// Normalize payload hash to hex format for canonical request
	// AWS SigV4 canonical requests always use hex-encoded SHA256
	normalizedPayload := normalizePayloadHash(hashedPayload)

	return &v4AuthInfo{
		Signature:     signV4Values.Signature,
		AccessKey:     signV4Values.Credential.accessKey,
		SignedHeaders: signV4Values.SignedHeaders,
		Date:          t,
		Region:        signV4Values.Credential.scope.region,
		Service:       signV4Values.Credential.scope.service,
		Scope:         signV4Values.Credential.getScope(),
		HashedPayload: normalizedPayload,
		IsPresigned:   false,
	}, s3err.ErrNone
}

func extractV4AuthInfoFromQuery(r *http.Request) (*v4AuthInfo, s3err.ErrorCode) {
	query := r.URL.Query()

	// Validate all required query parameters upfront for fail-fast behavior
	if query.Get("X-Amz-Algorithm") != signV4Algorithm {
		return nil, s3err.ErrSignatureVersionNotSupported
	}
	if query.Get("X-Amz-Date") == "" {
		return nil, s3err.ErrMissingDateHeader
	}
	if query.Get("X-Amz-Credential") == "" {
		return nil, s3err.ErrMissingFields
	}
	if query.Get("X-Amz-Signature") == "" {
		return nil, s3err.ErrMissingFields
	}
	if query.Get("X-Amz-SignedHeaders") == "" {
		return nil, s3err.ErrMissingFields
	}
	if query.Get("X-Amz-Expires") == "" {
		return nil, s3err.ErrInvalidQueryParams
	}
	signedHeaders, errCode := parseSignedHeaderList(query.Get("X-Amz-SignedHeaders"))
	if errCode != s3err.ErrNone {
		return nil, errCode
	}

	// Parse date
	dateStr := query.Get("X-Amz-Date")
	t, err := time.Parse(iso8601Format, dateStr)
	if err != nil {
		return nil, s3err.ErrMalformedDate
	}

	// Parse credential header
	credHeader, errCode := parseCredentialHeader("Credential=" + query.Get("X-Amz-Credential"))
	if errCode != s3err.ErrNone {
		return nil, errCode
	}

	// For presigned URLs, X-Amz-Content-Sha256 must come from the query parameter
	// (or default to UNSIGNED-PAYLOAD) because that's what was used for signing.
	// We must NOT check the request header as it wasn't part of the signature calculation.
	hashedPayload := query.Get("X-Amz-Content-Sha256")
	if hashedPayload == "" {
		hashedPayload = unsignedPayload
	}

	return &v4AuthInfo{
		Signature:     query.Get("X-Amz-Signature"),
		AccessKey:     credHeader.accessKey,
		SignedHeaders: signedHeaders,
		Date:          t,
		Region:        credHeader.scope.region,
		Service:       credHeader.scope.service,
		Scope:         credHeader.getScope(),
		HashedPayload: hashedPayload,
		IsPresigned:   true,
	}, s3err.ErrNone
}

func getCanonicalQueryString(r *http.Request, isPresigned bool) string {
	queryForCanonical := r.URL.Query()
	if isPresigned {
		queryForCanonical.Del("X-Amz-Signature")
	}
	for key := range queryForCanonical {
		sort.Strings(queryForCanonical[key])
	}
	return queryForCanonical.Encode()
}

func checkPresignedRequestExpiry(r *http.Request, t time.Time) s3err.ErrorCode {
	expiresStr := r.URL.Query().Get("X-Amz-Expires")
	// X-Amz-Expires is validated as required in extractV4AuthInfoFromQuery,
	// so it should never be empty here
	expires, err := strconv.ParseInt(expiresStr, 10, 64)
	if err != nil {
		return s3err.ErrMalformedDate
	}

	// The maximum value for X-Amz-Expires is 604800 seconds (7 days)
	// Allow 0 but it will immediately fail expiration check
	if expires < 0 {
		return s3err.ErrNegativeExpires
	}
	if expires > 604800 {
		return s3err.ErrMaximumExpires
	}

	expirationTime := t.Add(time.Duration(expires) * time.Second)
	if time.Now().UTC().After(expirationTime) {
		return s3err.ErrExpiredPresignRequest
	}
	return s3err.ErrNone
}

func (iam *IdentityAccessManagement) doesSignatureMatch(r *http.Request) (*Identity, string, s3err.ErrorCode) {
	identity, _, calculatedSignature, _, errCode := iam.verifyV4Signature(r, false)
	return identity, calculatedSignature, errCode
}

func (iam *IdentityAccessManagement) doesPresignedSignatureMatch(r *http.Request) (*Identity, string, s3err.ErrorCode) {
	identity, _, calculatedSignature, _, errCode := iam.verifyV4Signature(r, false)
	return identity, calculatedSignature, errCode
}

// credentialHeader data type represents structured form of Credential
// string from authorization header.
type credentialHeader struct {
	accessKey string
	scope     struct {
		date    time.Time
		region  string
		service string
		request string
	}
}

func (c credentialHeader) getScope() string {
	return strings.Join([]string{
		c.scope.date.Format(yyyymmdd),
		c.scope.region,
		c.scope.service,
		c.scope.request,
	}, "/")
}

// parse credentialHeader string into its structured form.
func parseCredentialHeader(credElement string) (ch credentialHeader, aec s3err.ErrorCode) {
	creds := strings.SplitN(strings.TrimSpace(credElement), "=", 2)
	if len(creds) != 2 {
		return ch, s3err.ErrMissingFields
	}
	if creds[0] != "Credential" {
		return ch, s3err.ErrMissingCredTag
	}
	credElements := strings.Split(strings.TrimSpace(creds[1]), "/")
	if len(credElements) != 5 {
		return ch, s3err.ErrCredMalformed
	}
	// Save access key id.
	cred := credentialHeader{
		accessKey: credElements[0],
	}
	var e error
	cred.scope.date, e = time.Parse(yyyymmdd, credElements[1])
	if e != nil {
		return ch, s3err.ErrMalformedCredentialDate
	}

	cred.scope.region = credElements[2]
	cred.scope.service = credElements[3] // "s3"
	cred.scope.request = credElements[4] // "aws4_request"
	return cred, s3err.ErrNone
}

// Parse signature from signature tag.
func parseSignature(signElement string) (string, s3err.ErrorCode) {
	signFields := strings.Split(strings.TrimSpace(signElement), "=")
	if len(signFields) != 2 {
		return "", s3err.ErrAuthorizationHeaderMalformed
	}
	if signFields[0] != "Signature" {
		return "", s3err.ErrMissingSignTag
	}
	if signFields[1] == "" {
		return "", s3err.ErrAuthorizationHeaderMalformed
	}
	signature := signFields[1]
	return signature, s3err.ErrNone
}

// Parse slice of signed headers from signed headers tag.
func parseSignedHeader(signedHdrElement string) ([]string, s3err.ErrorCode) {
	signedHdrFields := strings.Split(strings.TrimSpace(signedHdrElement), "=")
	if len(signedHdrFields) != 2 {
		return nil, s3err.ErrMissingFields
	}
	if signedHdrFields[0] != "SignedHeaders" {
		return nil, s3err.ErrMissingSignHeadersTag
	}
	return parseSignedHeaderList(signedHdrFields[1])
}

func parseSignedHeaderList(signedHeadersValue string) ([]string, s3err.ErrorCode) {
	if signedHeadersValue == "" {
		return nil, s3err.ErrMissingFields
	}
	signedHeaders := strings.Split(signedHeadersValue, ";")
	for _, header := range signedHeaders {
		if strings.TrimSpace(header) == "" {
			return nil, s3err.ErrMissingFields
		}
	}
	return signedHeaders, s3err.ErrNone
}

func (iam *IdentityAccessManagement) doesPolicySignatureV4Match(ctx context.Context, formValues http.Header) (*Identity, s3err.ErrorCode) {

	// Parse credential tag.
	credHeader, err := parseCredentialHeader("Credential=" + formValues.Get("X-Amz-Credential"))
	if err != s3err.ErrNone {
		return nil, err
	}

	identity, cred, found := iam.lookupByAccessKey(credHeader.accessKey)
	if !found {
		// Log detailed error information for InvalidAccessKeyId (POST policy)
		iam.m.RLock()
		availableKeyCount := len(iam.accessKeyIdent)
		iam.m.RUnlock()

		glog.Warningf("InvalidAccessKeyId (POST policy): attempted key '%s' not found. Available keys: %d, Auth enabled: %v",
			credHeader.accessKey, availableKeyCount, iam.isAuthEnabled)

		return nil, s3err.ErrInvalidAccessKeyID
	}

	// Check service account expiration
	if cred.isCredentialExpired() {
		glog.V(2).Infof("Service account credential %s has expired (expiration: %d, now: %d)",
			credHeader.accessKey, cred.Expiration, time.Now().Unix())
		return nil, s3err.ErrAccessDenied
	}

	// Get signing key.
	signingKey := getSigningKey(cred.SecretKey, credHeader.scope.date.Format(yyyymmdd), credHeader.scope.region, credHeader.scope.service)

	// Get signature.
	newSignature := getSignature(signingKey, formValues.Get("Policy"))

	// Verify signature.
	if !compareSignatureV4(newSignature, formValues.Get("X-Amz-Signature")) {
		return nil, s3err.ErrSignatureDoesNotMatch
	}
	s3_constants.RecordAccessKeyId(ctx, cred.AccessKey)
	return identity, s3err.ErrNone
}

// sigV4PayloadHashHeader is x-amz-content-sha256. It participates in the
// canonical request via the payload-hash line regardless of whether it is
// listed in SignedHeaders, so tampering with it still breaks the signature.
// It is therefore exempted from the "all x-amz-* headers must be signed" rule
// for both header-based and presigned requests.
const sigV4PayloadHashHeader = "x-amz-content-sha256"

// presignedSigV4ProtocolHeaders lists SigV4 protocol parameters that are
// conveyed in the presigned URL's query string. Some clients and proxies echo
// them as request headers; those duplicates do not influence signature
// computation or object persistence and are exempted from the
// "all x-amz-* headers must be signed" rule for presigned requests only.
var presignedSigV4ProtocolHeaders = map[string]struct{}{
	"x-amz-algorithm":     {},
	"x-amz-credential":    {},
	"x-amz-date":          {},
	"x-amz-expires":       {},
	"x-amz-signedheaders": {},
	"x-amz-signature":     {},
}

// presignedUnsignedSSECHeaders exempts SSE-C key material that AWS requires to
// be sent but not signed on presigned requests: only the algorithm header is
// part of the signature calculation.
var presignedUnsignedSSECHeaders = map[string]struct{}{
	strings.ToLower(s3_constants.AmzServerSideEncryptionCustomerKey):              {},
	strings.ToLower(s3_constants.AmzServerSideEncryptionCustomerKeyMD5):           {},
	strings.ToLower(s3_constants.AmzCopySourceServerSideEncryptionCustomerKey):    {},
	strings.ToLower(s3_constants.AmzCopySourceServerSideEncryptionCustomerKeyMD5): {},
}

// verifySignedHeadersCoverage rejects requests that carry x-amz-* headers
// outside of the SignedHeaders list. AWS SigV4 requires every x-amz-* header
// present in the request to be covered by the signature; without this check a
// presigned-URL holder can add unsigned headers (ACL, tagging, storage class,
// user metadata, encryption options, object lock) that the server still
// persists to object metadata.
func verifySignedHeadersCoverage(r *http.Request, signedHeaders []string, isPresigned bool) s3err.ErrorCode {
	signedSet := make(map[string]struct{}, len(signedHeaders))
	for _, h := range signedHeaders {
		signedSet[strings.ToLower(h)] = struct{}{}
	}
	for name := range r.Header {
		lower := strings.ToLower(name)
		if !strings.HasPrefix(lower, "x-amz-") {
			continue
		}
		if _, ok := signedSet[lower]; ok {
			continue
		}
		if lower == sigV4PayloadHashHeader {
			continue
		}
		if isPresigned {
			if _, exempt := presignedSigV4ProtocolHeaders[lower]; exempt {
				continue
			}
			if _, exempt := presignedUnsignedSSECHeaders[lower]; exempt {
				continue
			}
		}
		glog.V(2).Infof("reject %s %s: unsigned header %q not in SignedHeaders", r.Method, r.URL.Path, name)
		return s3err.ErrSignatureDoesNotMatch
	}
	return s3err.ErrNone
}

// Verify if extracted signed headers are not properly signed.
func extractSignedHeaders(signedHeaders []string, r *http.Request, externalHost string) (http.Header, s3err.ErrorCode) {
	reqHeaders := r.Header
	// If no signed headers are provided, then return an error.
	if len(signedHeaders) == 0 {
		return nil, s3err.ErrMissingFields
	}
	extractedSignedHeaders := make(http.Header)
	for _, header := range signedHeaders {
		// `host` is not a case-sensitive header, unlike other headers such as `x-amz-date`.
		if strings.ToLower(header) == "host" {
			// Get host value.
			hostHeaderValue := extractHostHeader(r, externalHost)
			extractedSignedHeaders[header] = []string{hostHeaderValue}
			continue
		}
		// The Go server strips Content-Length into r.ContentLength; clients do
		// sign it, so rebuild the value the same way the client saw it.
		if strings.EqualFold(header, "content-length") && r.ContentLength >= 0 {
			extractedSignedHeaders[header] = []string{strconv.FormatInt(r.ContentLength, 10)}
			continue
		}
		// For all other headers we need to find them in the HTTP headers and copy them over.
		// We skip non-existent headers to be compatible with AWS signatures.
		if values, ok := reqHeaders[http.CanonicalHeaderKey(header)]; ok {
			extractedSignedHeaders[header] = values
		}
	}
	return extractedSignedHeaders, s3err.ErrNone
}

// extractHostHeader returns the most likely host header value for signature verification.
func extractHostHeader(r *http.Request, externalHost string) string {
	return extractHostHeaderCandidates(r, externalHost)[0]
}

// extractHostHeaderCandidates returns the host values the client may have signed, most
// likely first. externalHost (from s3.externalUrl) leads when set, but the hosts derived
// from X-Forwarded-* headers or the request Host still follow it, so clients that reach
// the gateway directly rather than through the proxy keep verifying.
// When X-Forwarded-Host carries no port, the true client port is ambiguous: a proxy that
// kept the Host header makes the r.Host port right, one that rewrote it makes
// X-Forwarded-Port right, and a client on the scheme's default port signed no port at all.
func extractHostHeaderCandidates(r *http.Request, externalHost string) []string {
	var candidates []string
	if externalHost != "" {
		candidates = append(candidates, externalHost)
	}

	forwardedHost := r.Header.Get("X-Forwarded-Host")
	forwardedPort := r.Header.Get("X-Forwarded-Port")
	forwardedProto := r.Header.Get("X-Forwarded-Proto")

	// X-Forwarded-Proto and X-Forwarded-Port can be comma-separated lists when there are multiple proxies.
	// Use only the first value (first-hop).
	if comma := strings.Index(forwardedPort, ","); comma != -1 {
		forwardedPort = strings.TrimSpace(forwardedPort[:comma])
	}
	if comma := strings.Index(forwardedProto, ","); comma != -1 {
		forwardedProto = strings.TrimSpace(forwardedProto[:comma])
	}

	// Determine effective scheme for default port stripping.
	// Precedence: X-Forwarded-Proto > r.TLS > r.URL.Scheme > "http"
	scheme := "http"
	if r.URL.Scheme != "" {
		scheme = r.URL.Scheme
	}
	if r.TLS != nil {
		scheme = "https"
	}
	if forwardedProto != "" {
		scheme = forwardedProto
	}

	var host string
	var ports []string
	if forwardedHost != "" {
		// X-Forwarded-Host can be a comma-separated list of hosts when there are multiple proxies.
		// Use only the first host in the list and trim spaces for robustness.
		if comma := strings.Index(forwardedHost, ","); comma != -1 {
			host = strings.TrimSpace(forwardedHost[:comma])
		} else {
			host = strings.TrimSpace(forwardedHost)
		}

		// If the host itself contains a port, it should take precedence
		if h, p, err := net.SplitHostPort(host); err == nil {
			host = h
			ports = []string{p}
		} else {
			// SplitHostPort unbrackets IPv6 hosts, so unbracket the forwarded host to match
			if rh, rp, err := net.SplitHostPort(r.Host); err == nil && rh == strings.Trim(host, "[]") {
				ports = append(ports, rp)
			}
			if forwardedPort != "" {
				ports = append(ports, forwardedPort)
			}
			ports = append(ports, "")
		}
	} else {
		host = r.Host
		if host == "" {
			host = r.URL.Host
		}

		// If the host already contains a port, use it.
		// Otherwise, if X-Forwarded-Port is set, use it.
		if h, p, err := net.SplitHostPort(host); err == nil {
			host = h
			ports = []string{p}
		} else {
			if forwardedPort != "" {
				ports = append(ports, forwardedPort)
			}
			ports = append(ports, "")
		}
	}

	for _, port := range ports {
		candidate := joinSignedHost(host, port, scheme)
		if !slices.Contains(candidates, candidate) {
			candidates = append(candidates, candidate)
		}
	}
	return candidates
}

func bucketFromVirtualHost(host, domainConfig string) (string, bool) {
	if domainConfig == "" {
		return "", false
	}
	h := host
	if hh, _, err := net.SplitHostPort(host); err == nil {
		h = hh
	}
	h = strings.ToLower(h)
	pathStyleDomains, virtualHostDomains := classifyDomainNames(strings.Split(domainConfig, ","))
	for _, domain := range pathStyleDomains {
		if h == strings.ToLower(strings.TrimSpace(domain)) {
			return "", false
		}
	}
	for _, domain := range virtualHostDomains {
		suffix := "." + strings.ToLower(strings.TrimSpace(domain))
		if strings.HasSuffix(h, suffix) {
			bucket := h[:len(h)-len(suffix)]
			if bucket != "" {
				return bucket, true
			}
		}
	}
	return "", false
}

// joinSignedHost renders host:port the way AWS SDKs sign it: default ports are stripped
// to match SanitizeHostForHeader, and bare IPv6 addresses lose their brackets.
// Reference: https://github.com/aws/aws-sdk-go-v2/blob/main/aws/signer/internal/v4/host.go
func joinSignedHost(host, port, scheme string) string {
	if port != "" && !isDefaultPort(scheme, port) {
		// Strip existing brackets before calling JoinHostPort, which automatically adds
		// brackets for IPv6 addresses. This prevents double-bracketing like [[::1]]:8080.
		host = strings.Trim(host, "[]")
		return net.JoinHostPort(host, port)
	}

	if strings.Contains(host, ":") {
		return strings.Trim(host, "[]")
	}
	return host
}

// isDefaultPort returns true if the given port is the default for the scheme.
func isDefaultPort(scheme, port string) bool {
	switch port {
	case "80":
		return strings.EqualFold(scheme, "http")
	case "443":
		return strings.EqualFold(scheme, "https")
	default:
		return false
	}
}

// getScope generate a string of a specific date, an AWS region, and a service.
func getScope(t time.Time, region string, service string) string {
	scope := strings.Join([]string{
		t.Format(yyyymmdd),
		region,
		service,
		"aws4_request",
	}, "/")
	return scope
}

// getCanonicalRequest generate a canonical request of style
//
// canonicalRequest =
//
//	<HTTPMethod>\n
//	<CanonicalURI>\n
//	<CanonicalQueryString>\n
//	<CanonicalHeaders>\n
//	<SignedHeaders>\n
//	<HashedPayload>
func getCanonicalRequest(extractedSignedHeaders http.Header, payload, queryStr, urlPath, method string) string {
	rawQuery := strings.Replace(queryStr, "+", "%20", -1)
	encodedPath := encodePath(urlPath)
	canonicalRequest := strings.Join([]string{
		method,
		encodedPath,
		rawQuery,
		getCanonicalHeaders(extractedSignedHeaders),
		getSignedHeaders(extractedSignedHeaders),
		payload,
	}, "\n")
	return canonicalRequest
}

// getStringToSign a string based on selected query values.
func getStringToSign(canonicalRequest string, t time.Time, scope string) string {
	stringToSign := signV4Algorithm + "\n" + t.Format(iso8601Format) + "\n"
	stringToSign = stringToSign + scope + "\n"
	stringToSign = stringToSign + getSHA256Hash([]byte(canonicalRequest))
	return stringToSign
}

// getSHA256Hash returns hex-encoded SHA256 hash of the input data.
func getSHA256Hash(data []byte) string {
	hash := sha256.Sum256(data)
	return hex.EncodeToString(hash[:])
}

// sumHMAC calculate hmac between two input byte array.
func sumHMAC(key []byte, data []byte) []byte {
	hash := hmac.New(sha256.New, key)
	hash.Write(data)
	return hash.Sum(nil)
}

// getSigningKey hmac seed to calculate final signature.
func getSigningKey(secretKey string, time string, region string, service string) []byte {
	date := sumHMAC([]byte("AWS4"+secretKey), []byte(time))
	regionBytes := sumHMAC(date, []byte(region))
	serviceBytes := sumHMAC(regionBytes, []byte(service))
	signingKey := sumHMAC(serviceBytes, []byte("aws4_request"))
	return signingKey
}

// getCanonicalHeaders generate a list of request headers with their values
func getCanonicalHeaders(signedHeaders http.Header) string {
	var headers []string
	vals := make(http.Header)
	for k, vv := range signedHeaders {
		vals[strings.ToLower(k)] = vv
	}
	for k := range vals {
		headers = append(headers, k)
	}
	sort.Strings(headers)

	var buf bytes.Buffer
	for _, k := range headers {
		buf.WriteString(k)
		buf.WriteByte(':')
		for idx, v := range vals[k] {
			if idx > 0 {
				buf.WriteByte(',')
			}
			buf.WriteString(signV4TrimAll(v))
		}
		buf.WriteByte('\n')
	}
	return buf.String()
}

// signV4TrimAll trims leading and trailing spaces from each string in the slice, and trims sequential spaces.
func signV4TrimAll(input string) string {
	// Compress adjacent spaces (a space is determined by
	// unicode.IsSpace() internally here) to a single space and trim
	// leading and trailing spaces.
	return strings.Join(strings.Fields(input), " ")
}

// getSignedHeaders generate a string i.e alphabetically sorted, semicolon-separated list of lowercase request header names
func getSignedHeaders(signedHeaders http.Header) string {
	var headers []string
	for k := range signedHeaders {
		headers = append(headers, strings.ToLower(k))
	}
	sort.Strings(headers)
	return strings.Join(headers, ";")
}

// if object matches reserved string, no need to encode them
var reservedObjectNames = regexp.MustCompile("^[a-zA-Z0-9-_.~/]+$")

// pathHexTable is used for manual percent-encoding in encodePath to avoid
// the allocations of hex.EncodeToString + strings.ToUpper.
const pathHexTable = "0123456789ABCDEF"

// isPathUnreservedChar reports whether s is an RFC 3986 §2.3 unreserved
// character (or '/') that does not need percent-encoding in an object path.
func isPathUnreservedChar(s rune) bool {
	return 'A' <= s && s <= 'Z' ||
		'a' <= s && s <= 'z' ||
		'0' <= s && s <= '9' ||
		s == '-' || s == '_' || s == '.' || s == '~' || s == '/'
}

// encodePath encodes the strings from UTF-8 byte representations to HTML hex escape sequences
//
// This is necessary since regular url.Parse() and url.Encode() functions do not support UTF-8
// non english characters cannot be parsed due to the nature in which url.Encode() is written
//
// This function on the other hand is a direct replacement for url.Encode() technique to support
// pretty much every UTF-8 character.
func encodePath(pathName string) string {
	// Fast path: if every character is unreserved, the encoded form equals the
	// input, so return it unchanged with zero allocation. This is the common
	// case for ASCII object keys and avoids the regexp engine on the SigV4 hot
	// path, where encodePath runs on every authenticated request.
	needEncode := false
	for _, s := range pathName {
		if !isPathUnreservedChar(s) {
			needEncode = true
			break
		}
	}
	if !needEncode {
		return pathName
	}

	// Slow path: preallocated Builder + manual hex encoding.
	var buf strings.Builder
	buf.Grow(len(pathName) * 3) // encoded form is at most 3x the byte length
	for _, s := range pathName {
		if isPathUnreservedChar(s) { // §2.3 Unreserved characters (mark)
			buf.WriteRune(s)
		} else {
			runeLen := utf8.RuneLen(s)
			if runeLen < 0 {
				return pathName
			}
			var u [utf8.UTFMax]byte
			utf8.EncodeRune(u[:], s)
			for _, r := range u[:runeLen] {
				buf.WriteByte('%')
				buf.WriteByte(pathHexTable[r>>4])
				buf.WriteByte(pathHexTable[r&0x0f])
			}
		}
	}
	return buf.String()
}

// getSignature final signature in hexadecimal form.
func getSignature(signingKey []byte, stringToSign string) string {
	return hex.EncodeToString(sumHMAC(signingKey, []byte(stringToSign)))
}

// compareSignatureV4 returns true if and only if both signatures
// are equal. The signatures are expected to be hex-encoded strings
// according to the AWS S3 signature V4 spec.
func compareSignatureV4(sig1, sig2 string) bool {
	// The CTC using []byte(str) works because the hex encoding doesn't use
	// non-ASCII characters. Otherwise, we'd need to convert the strings to
	// a []rune of UTF-8 characters.
	return subtle.ConstantTimeCompare([]byte(sig1), []byte(sig2)) == 1
}